1) Field of the Invention
The present invention relates to a semiconductor optical amplifier and an optical signal processing apparatus, and more particularly to a semiconductor optical amplifier and an optical signal processing apparatus suitable for long-distance and broadband optical transmission and capable of high-speed operation.
2) Description of the Related Art
Wavelength division multiplexing (WDM) techniques have been developed recently as an optical transmission method to be used in a broadband optical network. Optical time division multiplexing (OTDM) techniques aiming at broadband optical transmission and time wavelength division multiplexing (TWDM) techniques combining WDM techniques and OTDM techniques have been proposed and are now under researches.
FIG. 1 is a conceptual diagram showing a broadband optical network. A plurality of nodes 1 are interconnected by optical channels (optical fibers) 2. Each node 1 performs optical signal regeneration, drop, add and routing. The optical signal regeneration is generally realized by the functions of amplifying, reshaping and retiming, and is called 3R regeneration.
In a conventional optical transmission method, each node converts once an optical signal into an electric signal, performs signal processing depending upon the state of the electric signal, and reconverts the processed electric signal into an optical signal. However, a response speed of an electric signal is lower than that of an optical signal because the response time is limited by the carrier mobility and CR time constants of electronic components. For example, the speed limit of signal processing using the method of converting an optical signal into an electric signal is 10 to 40 Gb/s. In order to perform signal processing at a speed higher than 10 to 40 Gb/s, it is essential to incorporate overall optical signal processing techniques by which an OTDM signal is processed always in the form of light.
FIG. 2 is a block diagram showing the structure of an optical node at which an OTDM signal is processed in the form of light. An optical node 1 is constituted of a regeneration function block 5, a drop function block 10, an add function block 20 and a routing function block 25.
An optical signal made of a plurality of time-division multiplex channels is input to an amplifier 6 of the regeneration function block 5. The amplifier 6 amplifies the optical signal. A reshaper 7 reshapes the waveform of the amplified optical signal. A retiming unit 8 corrects the time shifts of pulses of the waveform reshaped optical signal to recover regular positions of the pulses on the time axis. A clock sampler 9 samples a clock signal from the amplified optical signal and supplies the clock signal to each unit of the optical node.
The optical signal subjected to the retiming process is input to a pulse branching unit 11 in the drop function block 10 and to an optical gate 21 in the add function block 20. The optical signal branched by the pulse branching unit 11 is input to a header reader 12 and a delay memory 13. The header reader 12 reads header information of the optical signal for each channel. The delay memory 13 delays the optical signal by a predetermined time. A demultiplexer 14 demultiplexes the delayed optical signal of each channel to pick up the optical signal corresponding to the channels to be dropped at the node.
The optical gate 21 in the add function block 20 sets an empty state to a time slot of each channel dropped at the node from the optical signal subjected to the retiming process. A multiplexer 22 multiplexes an optical signal to be added at the node. The multiplex optical signal is added to the empty time slot of the optical signal passing through the optical gate 21. A header former 23 forms a header of the optical signal added at the node, and adds the header information to a predetermined time slot of the optical signal passing through the optical gate 21.
The optical signal added with the header information is input to a wavelength converter 24 and to the routing function block 25. The wavelength converter 24 converts the wavelength of the input optical signal, and inputs the wavelength converted optical signal to the routing function block 25. The routing function block 25 distributes the optical signal of each channel to the next node in accordance with the routing information of the input optical signal of each channel.
Among those elements realizing the above-described functions of the optical node, it is desired to realize an element having the two functions of the optical amplifier 6 and waveform reshaper 7 (the element having the two functions is called a 2R element) and the elements having the functions of the optical demultiplexer 14 for demultiplexing the optical signal of a plurality of time-division multiplex channels and the wavelength converter 24.
Such functions can be realized by a semiconductor optical amplifier (SOA).
FIG. 15 is a perspective view showing the outline of a conventional SOA. This SOA has the structure that an active layer 200 having an optical amplification gain is sandwiched between a p-type semiconductor layer 201 and an n-type semiconductor layer 202. The active layer 200 is a quantum well layer or a semiconductor layer made of semiconductor having a band gap smaller than those of the semiconductor layers on both sides of the active layer 200.
As a forward bias is applied to the active layer 200, the carrier distribution in the active layer 200 becomes a reversed distribution state. As an optical signal 203 is incident upon the active layer 200 from one end thereof, the optical signal is amplified in the active layer 200 and outputs from the opposite end. Next, with reference to FIGS. 16 and 17, the operation principle of a semiconductor optical amplifier applied to an optical signal processing apparatus will be described.
FIG. 16 illustrates a semiconductor optical amplifier operating as a 2R element. A long-distance-transmitted optical signal is input to the semiconductor optical amplifier 210. The pulse intensities of an optical signal are irregular because of various factors during transmission, such as generation of noises, external disturbance of a transmission system, and branch. The optical intensity relation between input and output signals of the semiconductor optical amplifier 210 has saturation characteristics. This is called gain saturation.
If the optical signal having irregular pulse intensities is input to the semiconductor optical amplifier 210 having such gain saturation, the optical signal is amplified and the pulse intensities of the output signal become approximately uniform. Namely, the semiconductor optical amplifier 210 has the functions of optical amplifying and waveform reshaping.
FIG. 17 illustrates an operation of a semiconductor optical amplifier as a wavelength converter. An optical signal sig1 having a wavelength xcex1 and an optical signal sig2 having a wavelength xcex2 are input to the semiconductor optical amplifier 210. The optical signal sig1 is an optical pulse train and the optical signal sig2 is continuous light. The intensities of the optical signals and the amplification characteristics of the semiconductor optical amplifier 210 are adjusted so that the gain of the semiconductor optical amplifier 210 saturates when both the optical signals sig1 and sig2 are input.
Since the on-pulse and off-pulse of the optical signal sig1 change the gain of the optical signal sig2, the intensity of the optical signal sig2 is modulated. Therefore, the optical signal sig2 having the wavelength xcex2 output from the semiconductor optical amplifier 210 has the inverted waveform of the input optical signal sig1. Namely, it means that the optical signal sig1 having the wavelength xcex1 is converted into the optical signal sig2 having the wavelength xcex2.
As described above, a semiconductor optical amplifier can be used as a 2R element or a wavelength converter. However, the conventional semiconductor optical amplifier has a low response speed and the processible frequency is about 1 Gb/s at most. The reason for this will be described in the following.
The rate equation of an optical pulse S (z, T) propagating in a semiconductor optical amplifier is represented by:                                           ∂                          S              ⁡                              (                                  z                  ,                  T                                )                                                          ∂            z                          =                              (                                          Γ                ⁢                                  xe2x80x83                                ⁢                g                            -                              α                loss                                      )                    ⁢                      S            ⁡                          (                              z                ,                T                            )                                                          (        1        )            
where z is the coordinate value of a propagation direction of light in the optical wave guide of a semiconductor optical amplifier, T is time, g is a gain of the active layer of the semiconductor optical amplifier, xcex1loss is a loss in the optical wave guide, and xcex93 is a light confinement coefficient. The gain g of the active layer is determined by the carrier density. The rate equation of a carrier density N is given by:                                                                                           ⅆ                  N                                                  ⅆ                  T                                            =                                                                                          N                      0                                        -                    N                                                        τ                    r                                                  -                                  Γ                  ⁢                                      xe2x80x83                                    ⁢                                      S                    ⁡                                          (                                              z                        ,                        T                                            )                                                        ⁢                  g                                                                                                                        N                0                            =                                                                    τ                    r                                    ⁢                  J                                ed                                                                        (        2        )            
where xcfx84r is a recombination time of carriers, J is a current density, d is a thickness of the active layer, and e is an electron charge. According to the evaluation experiments made by the present inventor, the gain g can be approximated to:
g=A(Lxe2x88x92Lz)2+B(Lxe2x88x92Lz)3xe2x80x83xe2x80x83(3) 
where L is a length of the semiconductor optical amplifier and each constant is defined as in the following:
A=3P/(Lzxe2x88x92Lp)2 
B=2P/(Lzxe2x88x92Lp)3 
Lz=Lzoxe2x88x92C0(Nxe2x88x92N0) 
Lp=L0xe2x88x92B0(Nxe2x88x92N0) 
P=A0(Nxe2x88x92N0)+A0N0exp(xe2x88x92N/N0) 
A0=3.0xc3x9710xe2x88x9216 [cm2]
B0=3.0xc3x9710xe2x88x9220 [xcexcmxc2x7cm3]
C0=xe2x88x923.0xc3x9710xe2x88x9221 [xcexcmxc2x7cm3]
Lzo=1.65[xcexcm]
L0=1.60[xcexcm]
N0=7.0xc3x971017 [cmxe2x88x923]xe2x80x83xe2x80x83(4) 
By substituting the equation (3) into the equation (2), a differential equation of the carrier density N can be obtained. By solving this differential equation, a graph of the carrier density N changing with time can be obtained. By substituting the carrier density N changing with time into the equation (3), a graph of the gain g changing with time can be obtained.
FIG. 18 shows the calculation results of changes in the gain g with time after an optical pulse having a pulse width of 10 ps is input to a semiconductor optical amplifier starting from time 0. The abscissa represents a lapse time after the time 0 in the unit of xe2x80x9cpsxe2x80x9d, and the ordinate represents the gain g in the unit of cmxe2x88x921. The active layer of the semiconductor optical amplifier used for evaluation was made of InGaAsP corresponding to a band gap of 1.55 xcexcm and an operation temperature was 295 K. In FIG. 18, the gains g are shown at the current densities in the semiconductor optical amplifier of 4 k/cm2, 8 k/cm2 and 16 kA/cm2. While the optical pulse is input, the gain g lowers because the carrier density is lowered by optical transition of electrons in the conduction band.
FIG. 19 shows a change in the gain g with time after an optical pulse having a pulse width of 10 ps is input to a semiconductor optical amplifier at a bit rate of 40 Gb/s. The abscissa represents a lapse time in the unit of xe2x80x9cnsxe2x80x9d, and the ordinate represents the gain g in the unit of cmxe2x88x921. The structure of the active layer and the operation temperature were the same as those in the case of FIG. 18. The current density is 8 kA/cm2. While the optical pulse is input, the gain g lowers. During the period while the optical pulse is not input, the gain g increases because carriers are supplied to the active layer.
It can be seen from this graph that the gain change cannot follow the optical signal of 40 Gb/s because of a low response speed of the gain. It can be seen from the graph of FIG. 19 that the response time of the gain g is about 0.3 ns. It can be understood from this response time that the operation frequency of a conventional semiconductor optical amplifier is 2 Gb/s at most.
FIG. 20 shows the light intensities of input and output signals when an optical signal having a pulse width of 10 ps is input to a conventional semiconductor optical amplifier at a bit speed of 40 Gb/s. The abscissa represents a lapse time in the unit of xe2x80x9cpsxe2x80x9d, and the ordinate represents the optical output in the unit of xe2x80x9cmWxe2x80x9d. The structure of the semiconductor optical amplifier was similar to that of FIG. 18, the current density was 8 kA/cm2, the operation temperature was 295 K, and the active layer length was 600 xcexcm. Since the response characteristics of the gain are poor, the peak power of each pulse of the output signal gradually lowers and it takes a long time for the peak power to become constant. Such a change in the peak power may cause a transmission error.
Next, with reference to FIGS. 21A and 21B, the operation principle and an example of structure of an optical signal separator unit (demultiplexer) using a conventional semiconductor optical amplifier will be described.
As shown in FIG. 21A, an optical control signal sig3 and an optical signal sig4 are input to a semiconductor optical amplifier 210. For example, the optical signal sig4 is a signal of time-division multiplex channels #1 to #4. A pulse of the optical control signal sig3 is input to the semiconductor optical amplifier 210 synchronously with the time slot of the channel #2 of the optical signal sig4.
As the pulse of the control signal sig3 is superposed upon the pulse of the optical signal sig4, the carrier density of the active layer of the semiconductor optical amplifier 210 lowers greatly and the refraction factor changes. Therefore, the phase of the pulse of the channel #2 of the optical signal sig4 changes. In FIG. 21A, the pulses having the changed phase are shown hatched. The semiconductor optical amplifier 210 is adjusted so that the phase change becomes just xcfx80.
Since the response characteristics of a semiconductor optical amplifier are poor, the pulse of the control signal sig3 input synchronously with the time slot of the channel #2 of the optical signal sig4 also influences the time slots of the channels #3 and #4 of the optical signal sig4. From this reason, the phases of the pulses of the channels #3 and #4 are also changed.
FIG. 21B shows an example of the structure of an optical signal separator capable of solving the above-described problem. A first semiconductor optical amplifier 210A is inserted into one optical path of a Mach-Zehnder type interferometer, and a second semiconductor optical amplifier 210B is inserted into the other optical path. An optical signal sig4 obtained by time-division multiplexing four channels is input to both the semiconductor optical amplifiers 210A and 210B.
An optical control signal sig3 is input to the first semiconductor optical amplifier 210A, and a delayed optical control signal sig3 is input to the second semiconductor optical amplifier 210B. The control signal sig3 input to the first semiconductor optical amplifier 210A has a pulse corresponding in time to the time slot of the channel #2 of the optical signal sig4. The optical signal sig3 input to the second semiconductor optical amplifier 210B was delayed and corresponds in time to the time slot of the channel #3 of the optical signal sig4.
In the first semiconductor optical amplifier 210A, since the pulse of the control signal sig3 influences the pulses of the channels #2 to #4, the phases of the pulses of the channels #2 to #4 of an output optical signal sig5 change. In the second semiconductor optical amplifier 210B, the pulse of the control signal sig3 influences the pulses of the channels #3 and #4, the phases of the pulses of the channels #3 and #4 of an output optical signal sig6 change. In FIG. 21B, the pulses having the changed phases are shown hatched.
Since the optical signals sig5 and sig6 interfere with each other, the pulses of the channels #1, #3 and #4 are output from one output optical wave guide 211A of the Mach-Zehnder type interferometer, and the pulse of the channel #2 is output from the other output optical wave guide 211B. In this manner, a signal of a desired channel can be separated from the time-division multiplex optical signal.
As described above, it is difficult for an optical signal processing apparatus using a conventional semiconductor optical amplifier to realize high speed transmission at about 2 Gb/s or higher.
In the optical signal separator unit shown in FIG. 21B, the control signal sig3 is delayed and then input to the second semiconductor optical amplifier 210B to compensate for the poor response characteristics of the second semiconductor optical amplifier 210B. However, the optical signal separator unit using this compensation method has a complicated structure, requires a fine adjustment, and is difficult to perform a stable operation.
It is an object of the present invention to provide an optical signal processing method and an optical signal processing apparatus capable of performing a high speed and stable operation.
According to one aspect of the present invention, there is provided an optical signal processing method, comprising the steps of: injecting carriers into a plurality of quantum dots by applying a bias voltage to a semiconductor region of a semiconductor optical amplifier, the plurality of quantum dots for three-dimensionally confining carriers being distributed in the semiconductor region; and amplifying an optical pulse signal having a bit rate of 2 Gb/s or higher input to the semiconductor optical amplifier by generating induced emission by optical transition of the carriers in the quantum dots.
According to another aspect of the present invention, there is provided an optical signal processing method, comprising the steps of: injecting carriers into a plurality of quantum dots by applying a bias voltage to a semiconductor region of a semiconductor optical amplifier, the plurality of quantum dots for three-dimensionally confining carriers being distributed in the semiconductor region; and inputting an optical pulse signal having a first wavelength and continuous light having a second wavelength different from the first wavelength to the semiconductor optical amplifier, to make an output waveform of the continuous light change with a waveform of the optical pulse signal by lowering a gain of the semiconductor optical amplifier upon input of an optical pulse of the optical pulse signal.
According to another aspect of the present invention, there is provided an optical signal processing method, comprising the steps of: injecting carriers into a plurality of quantum dots by applying a bias voltage to semiconductor regions of first and second semiconductor optical amplifiers, the plurality of quantum dots for three-dimensionally confining carriers being distributed in each of the semiconductor regions; inputting same multiplex signals obtained by time-division multiplexing a plurality of optical pulse signals to both of the first and second semiconductor optical amplifiers and inputting an optical control signal only to the first semiconductor optical amplifier synchronously with a time slot corresponding to one the optical pulse signals constructing the multiplex signal; and making the multiplex signals output from the first and second semiconductor optical amplifiers interfere with each other to separate one optical signal from the multiplex signal.
By utilizing optical transition of carriers in quantum dots, a response speed of the semiconductor optical amplifier can be improved.
According to another aspect of the present invention, there is provided an optical signal processing apparatus comprising: a semiconductor optical amplifier having a semiconductor region, a plurality of quantum dots for three-dimensionally confining carriers being distributed in the semiconductor region; and an optical input device for inputting an optical pulse signal having a first wavelength and continuous light having a second wavelength different from the first wavelength, to said semiconductor optical amplifier.
When a pulse of the optical pulse signal having the first wavelength is input, a gain saturation occurs so that the gain of the continuous light having the second wavelength lowers. The output intensity of the optical signal having the second wavelength changes with the waveform of the optical pulse signal having the first wavelength. As a result, the first wavelength is converted into the second wavelength.
According to another aspect of the present invention, there is provided an optical signal processing apparatus, comprising: first and second semiconductor optical amplifiers each having a semiconductor region, a plurality of quantum dots for three-dimensionally confining carriers being distributed in the semiconductor region; a first optical unit for inputting same multiplex signals obtained by time-division multiplexing a plurality of optical pulse signals to both of the first and second semiconductor optical amplifiers; a second optical unit for inputting an optical control pulse signal only to the first semiconductor optical amplifier synchronously with a time slot corresponding to one of the optical pulse signals constructing the multiplex signal; and an interferometer for making multiplex signals output from the first and second semiconductor optical amplifiers interfere with each other to separate one of the optical pulse signals from the multiplex signal.
According to another aspect of the present invention, there is provided an optical signal processing apparatus, comprising: first and second semiconductor optical amplifiers each having a semiconductor region, a plurality of quantum dots for three-dimensionally confining carriers being distributed in the semiconductor region; a first optical unit for inputting an optical pulse signal to the first and second semiconductor optical amplifiers; a second optical unit for inputting an optical control pulse signal only to the first semiconductor optical amplifier; and an interferometer for making optical pulse signals output from the first and second semiconductor optical amplifiers interfere with each other to separate an optical pulse from the optical pulse signals, the optical pulse being synchronous with the optical control pulse signal.
By utilizing optical transition of carriers in quantum dots, a response speed of the semiconductor optical amplifier can be improved. It is therefore possible to change the phase of a pulse of a desired time slot, by inputting an optical control signal. Since the low response speed of the semiconductor optical amplifier is not necessary to be compensated, the optical control signal is not necessary to be input to the second semiconductor optical amplifier so that the apparatus structure can be simplified. Fine adjustment of an operation point is not necessary.
According to another aspect of the present invention, there is provided an optical signal processing apparatus comprising: an optical absorbing and saturating element for absorbing a component of incident light having an intensity equal to or lower than a threshold value and allowing a component through, having an intensity higher than the threshold value; a first optical unit for inputting an optical pulse signal to said optical absorbing and saturating element; and a semiconductor optical amplifier having a semiconductor region, an optical pulse signal output from said optical absorbing and saturating element is input to said semiconductor optical amplifier, and a plurality of quantum dots for three-dimensionally confining carriers being distributed in the semiconductor region.
The optical absorbing and saturating element can eliminate the background noises of incidence light. An S/N ratio of an amplified optical signal can be improved.
According to another aspect of the present invention, there is provided an optical signal processing apparatus comprising: a plurality of optical absorbing and saturating elements each for absorbing a component of incidence light having an intensity equal to or lower than a threshold value and allowing a component through, having an intensity higher than the threshold value; a plurality of semiconductor optical amplifiers each having a semiconductor region, a plurality of quantum dots for three-dimensionally confining carriers being distributed in the semiconductor region; and optical wave guides for connecting said plurality of optical absorbing and saturating elements and semiconductor optical amplifiers in tandem.
The waveform reshaping effects can be improved by connecting the plurality of optical absorbing and saturating elements and semiconductor optical amplifiers in tandem.
An optical signal processing apparatus having a high response speed can be obtained by applying a high speed semiconductor optical amplifier to an optical signal amplifying and reshaping unit, a wavelength converter unit or an optical signal separator unit.